This invention relates to shadow mask type cathode ray tubes and, particularly to contours of shadow masks within such tubes.
In a shadow mask tube, a plurality of convergent electron beams are projected through a multi-apertured color selection shadow mask to a mosaic screen. The beam paths are such that each beam impinges upon and excites only one kind of color-emitting phosphor on the screen. Generally, the shadow mask is attached to a rigid frame, which in turn, is suspended within the picture tube envelope.
Presently, all commercial color picture tubes have front or viewing faceplates that are either domed or cylindrical. However, it is desirable to develop a tube having a generally flat faceplate. There are problems that must be solved before a tube having a flat faceplate is commercially feasible. A major problem involves the shadow mask. According to prior art tube design concepts, in tubes having curved faceplates, the shadow mask is similarly curved so that it somewhat parallels the faceplate contour. Thus, in keeping with these prior art concepts, in a tube with a flat faceplate, the corresponding shadow mask should also have an almost flat contour. However, such a mask has insufficient self-supporting strength or rigidity. One way to provide this strength or ridigity would be to put the mask under tension as is done in some commercially available tubes having cylindrical faceplates. However, tension methods require undesirably expensive frame structures. Another way of providing strength to the mask is to give it some degree of contour such as by corrugating it, as suggested in U.S. Pat. No. 4,072,876 issued to A. M. Morrell on Feb. 7, 1978. It has been found, however, that a regular corrugated shape with a substantially sine wave cross section may be somewhat less than an optimum contour. For example, in a tube having a corrugated mask, aperture-to-aperture spacing and aperture width vary as functions of both mask-to-screen spacing and the relative angle formed between the electron beams and the mask. The required variations in aperture width create substantial problems in etching apertures into the mask. Since the required variations will be greatest at the edges of the mask, it is desirable to either decrease the mask-to-screen variations or decrease the beam-mask angle at these edges. The present invention therefore provides differing shadow mask contours that may be utilized to solve or at least reduce the foregoing and other various problems occurring in tubes with substantially flat faceplates.